Search Results
Abstract
Scaling of the turbulent energy equation suggests the balance of terms in the ocean is between turbulent production, dissipation and the loss to buoyancy. In this paper two models for the source of oceanic turbulence are considered; namely, production by the Reynolds stress working against a time variable mean shear, and the gravitational collapse of Kelvin-Helmholtz instabilities. Both of these shear instabilities are believed to be important in the ocean. Using values for the critical flux Richardson number and the measurements from studies of Kelvin-Helmholtz instabilities, the efficiency of turbulent mixing is shown to be comparable for the two models. Therefore, a general relationship between the dissipation rate and the buoyancy flux due to the small-scale turbulent velocity fluctuations is derived. The result is expressed as an upper bound on the value of the turbulent eddy coefficient for mass K ρ ⩽ 0.2ε/N 2. Values of K ρ are calculated from recent oceanic measurements of energy dissipation. Isopycnal advection and doubly diffusive phenomena are not included in the model.
Abstract
Scaling of the turbulent energy equation suggests the balance of terms in the ocean is between turbulent production, dissipation and the loss to buoyancy. In this paper two models for the source of oceanic turbulence are considered; namely, production by the Reynolds stress working against a time variable mean shear, and the gravitational collapse of Kelvin-Helmholtz instabilities. Both of these shear instabilities are believed to be important in the ocean. Using values for the critical flux Richardson number and the measurements from studies of Kelvin-Helmholtz instabilities, the efficiency of turbulent mixing is shown to be comparable for the two models. Therefore, a general relationship between the dissipation rate and the buoyancy flux due to the small-scale turbulent velocity fluctuations is derived. The result is expressed as an upper bound on the value of the turbulent eddy coefficient for mass K ρ ⩽ 0.2ε/N 2. Values of K ρ are calculated from recent oceanic measurements of energy dissipation. Isopycnal advection and doubly diffusive phenomena are not included in the model.
Abstract
Measurements of the vertical component of the temperature gradient in Powell Lake show phenomena similar to those observed in the ocean. The gradient is an irregular function of depth, with temperature inversions indicating static instability of the water column on the centimeter scale.
The lower portion of the lake contains old sea water. Doubly diffusive layers were found only near the very bottom of the lake and another form of thermohaline circulation may exist for 60 m above the layers.
Abstract
Measurements of the vertical component of the temperature gradient in Powell Lake show phenomena similar to those observed in the ocean. The gradient is an irregular function of depth, with temperature inversions indicating static instability of the water column on the centimeter scale.
The lower portion of the lake contains old sea water. Doubly diffusive layers were found only near the very bottom of the lake and another form of thermohaline circulation may exist for 60 m above the layers.
Abstract
Vertical profiles of temperature microstructure were collected at seven sites in the equatorial Atlantic between 24°W and 33°W, 2°N and 1°20′S. The use of three identical temperature microstructure profiles gives insight into the spatial and temporal variation of the temperature microstructure. Data on the velocity microstructure taken with a fourth instrument show a relationship between temperature and velocity microstructure.
Cox numbers show a relative minimum near the center of the core with largest values in the shear region between the South Equatorial Current and the Equatorial Undercurrent.
Abstract
Vertical profiles of temperature microstructure were collected at seven sites in the equatorial Atlantic between 24°W and 33°W, 2°N and 1°20′S. The use of three identical temperature microstructure profiles gives insight into the spatial and temporal variation of the temperature microstructure. Data on the velocity microstructure taken with a fourth instrument show a relationship between temperature and velocity microstructure.
Cox numbers show a relative minimum near the center of the core with largest values in the shear region between the South Equatorial Current and the Equatorial Undercurrent.
Abstract
A series of profiles of velocity microstructure along 152°E in the western North Pacific Ocean were collected in May–June 1982. Large, averaged turbulent dissipation rates, ε, found in the main thermocline (400 to 1000 m) were determined by a combination of large independent estimates of ε and a greater rate of occurrence of turbulent events in the main thermocline than elsewhere. Concurrently we find that averaged values of ε exhibit a positive dependence on the buoyancy frequency, N, and that form ε = aN γ is best fit by γ = 1 when only the data below 400 m are considered. Of the more than 5000 m of data collected below 1000 m depth, 12% showed measurable turbulence and dominated the depth averages. A deep ocean estimate of an upper bound to the eddy coefficient for vertical diffusion, K ρ, is 10−4 m2 s−1 and not significantly different from the value estimated by Munk. The inferred dependence of the mass flux with depth indicates the relative significance of vertical mixing in the main thermocline. Other processes must influence the maintenance of the more weakly stratified 15°–18°C water above.
Abstract
A series of profiles of velocity microstructure along 152°E in the western North Pacific Ocean were collected in May–June 1982. Large, averaged turbulent dissipation rates, ε, found in the main thermocline (400 to 1000 m) were determined by a combination of large independent estimates of ε and a greater rate of occurrence of turbulent events in the main thermocline than elsewhere. Concurrently we find that averaged values of ε exhibit a positive dependence on the buoyancy frequency, N, and that form ε = aN γ is best fit by γ = 1 when only the data below 400 m are considered. Of the more than 5000 m of data collected below 1000 m depth, 12% showed measurable turbulence and dominated the depth averages. A deep ocean estimate of an upper bound to the eddy coefficient for vertical diffusion, K ρ, is 10−4 m2 s−1 and not significantly different from the value estimated by Munk. The inferred dependence of the mass flux with depth indicates the relative significance of vertical mixing in the main thermocline. Other processes must influence the maintenance of the more weakly stratified 15°–18°C water above.
Abstract
Observations are described in an experiment undertaken to determine the response of a stratified inlet to changing conditions of wind, tide and runoff. Time series of conductivity profiles taken in Alberni Inlet, British Columbia, show marked fluctuations in surface layer thickness that appear to be related to strong winds. The effect of an up-inlet wind is to produce a rapid thickening of the fresh-water layer at the inlet head which may persist for several days. Strong winds were also associated with significant changes in the intensity of stratification.
Abstract
Observations are described in an experiment undertaken to determine the response of a stratified inlet to changing conditions of wind, tide and runoff. Time series of conductivity profiles taken in Alberni Inlet, British Columbia, show marked fluctuations in surface layer thickness that appear to be related to strong winds. The effect of an up-inlet wind is to produce a rapid thickening of the fresh-water layer at the inlet head which may persist for several days. Strong winds were also associated with significant changes in the intensity of stratification.
Abstract
High-resolution velocity shear, CTD, and microstructure measurements were made simultaneously from the research submarine Dolphin in Monterey Bay in October 1984. During three consecutive dives, the Dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of Monterey. Inside the seasonal thermocline, the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally. The thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline. In contrast, no large shear layers were observed below a 50 m depth. The depth levels at which the shear layers were observed were nearly constant throughout each dive, suggesting that the shear layers, with some unknown degree of intermittency, might extend horizontally over several square kilometers. The shear vector inside the seasonal themocline (at σ t = 25.5) rotated 360° over an inertial period, but did appear to propagate vertically over the 30-hour observation period. These observations suggest that the passage of a storm caused the upper thermocline to ring, creating a local jetlike flow below the mixed layer.
Abstract
High-resolution velocity shear, CTD, and microstructure measurements were made simultaneously from the research submarine Dolphin in Monterey Bay in October 1984. During three consecutive dives, the Dolphin cycled between the surface and 110 m along predetermined tracks 10 miles northwest of Monterey. Inside the seasonal thermocline, the vertical velocity shear appeared to be concentrated in layers 10 m thick extending several kilometers horizontally. The thickness of the shear layers is consistent with the typical size of turbulent patches encountered in the seasonal thermocline. In contrast, no large shear layers were observed below a 50 m depth. The depth levels at which the shear layers were observed were nearly constant throughout each dive, suggesting that the shear layers, with some unknown degree of intermittency, might extend horizontally over several square kilometers. The shear vector inside the seasonal themocline (at σ t = 25.5) rotated 360° over an inertial period, but did appear to propagate vertically over the 30-hour observation period. These observations suggest that the passage of a storm caused the upper thermocline to ring, creating a local jetlike flow below the mixed layer.
Abstract
Observations of turbulent energy dissipation rate ε in the deep surface mixed layer at a mid-Sargasso site are presented: two occupations of this site include a large range of local meteorological forcing. Two frontal passages and a large time interval between profiles during the first series of measurements preclude examination of the turbulent kinetic energy balance: qualitatively, a profile taken during the strongest wind-wave forcing of the observation set suggests that layer deepening was not being driven directly from the surface, but by a shear instability at the mixed layer base. A quantitative assessment of terms in the steady-state locally balanced model of the turbulent kinetic energy budget proposed by Niiler (1975) has been possible for two profiles having dissipation characteristics and surface meteorological conditions which allow us to argue for the absence of all but a few of the possible source/sink terms in the turbulent kinetic energy balance. In one case, a steady-state local balance is possible. In the other case, a local balance can be maintained by giving up the steady-state assumption. i.e., by including the time rate of decay of the turbulent kinetic energy. Other possible balances exist. The analysis of the surface mixed-layer turbulent kinetic energy balance highlights two major uncertainties-parameterization of the wind-wave forcing term and lack of reliable dissipation measurements in the upper 10–20 m of the water column.
Abstract
Observations of turbulent energy dissipation rate ε in the deep surface mixed layer at a mid-Sargasso site are presented: two occupations of this site include a large range of local meteorological forcing. Two frontal passages and a large time interval between profiles during the first series of measurements preclude examination of the turbulent kinetic energy balance: qualitatively, a profile taken during the strongest wind-wave forcing of the observation set suggests that layer deepening was not being driven directly from the surface, but by a shear instability at the mixed layer base. A quantitative assessment of terms in the steady-state locally balanced model of the turbulent kinetic energy budget proposed by Niiler (1975) has been possible for two profiles having dissipation characteristics and surface meteorological conditions which allow us to argue for the absence of all but a few of the possible source/sink terms in the turbulent kinetic energy balance. In one case, a steady-state local balance is possible. In the other case, a local balance can be maintained by giving up the steady-state assumption. i.e., by including the time rate of decay of the turbulent kinetic energy. Other possible balances exist. The analysis of the surface mixed-layer turbulent kinetic energy balance highlights two major uncertainties-parameterization of the wind-wave forcing term and lack of reliable dissipation measurements in the upper 10–20 m of the water column.
Abstract
Turbulence measurements from the central equatorial Pacific in February 1982 have been analyzed and compared to synoptic CTD and current velocity profiles and current meter data. These suggest considerably more time (if not space) variability than had previously been anticipated. Above 300 m at the equator the turbulence levels were greater but less than previous equatorial measurements, and turbulent patches occurred more frequently than elsewhere in the open ocean. Below 300 m the occurrence of turbulent patches was less frequent than in other regions of the ocean, except for the persistence of a patch at 500 m.
Abstract
Turbulence measurements from the central equatorial Pacific in February 1982 have been analyzed and compared to synoptic CTD and current velocity profiles and current meter data. These suggest considerably more time (if not space) variability than had previously been anticipated. Above 300 m at the equator the turbulence levels were greater but less than previous equatorial measurements, and turbulent patches occurred more frequently than elsewhere in the open ocean. Below 300 m the occurrence of turbulent patches was less frequent than in other regions of the ocean, except for the persistence of a patch at 500 m.
Abstract
Turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 × 20 cm2. Measurements are performed in the New York Bight at elevations ranging from 10 cm to about 1.4 m above the seafloor. Recorded data for each elevation consists of 130 s of image pairs recorded at 1 Hz. After processing, the data at each elevation consist of 130 instantaneous spatial velocity distributions within the sample area. The vertical distribution of mean velocity indicates the presence of large-scale shear even at the highest measurement station. The flow also undergoes variations at timescales longer than the present data series.
Spatial spectra of the energy and dissipation are calculated from individual vector maps. The data extend well beyond the peak in the dissipation spectrum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range. The vector maps are also patched together to generate extended velocity distributions using the Taylor hypothesis. Spectra calculated from the extended data cover about three decades in wavenumber space. For the overlapping range the extended spectra show small differences from those determined using the instantaneous distributions. Use of the Taylor hypothesis causes “contamination” of the extended spectra with surface waves. Nevetheless, the results still indicate that the turbulence is also anisotropic at low wavenumbers (energy containing eddies). The vertical component of velocity fluctuations at energy containing scales is significantly damped as the bottom is approached, while the horizontal component maintains a similar energy level at all elevations.
Different methods of estimating the turbulent energy dissipation are compared. Several of these methods are possible only with 2D data, such as that provided by PIV, including a “direct” method, which is based on measured components of the deformation tensor. Estimates based on assumptions of isotropy are typically larger than those based on the direct method (using available velocity gradients and least number of assumptions), but the differences vary from 30% to 100%.
Abstract
Turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 × 20 cm2. Measurements are performed in the New York Bight at elevations ranging from 10 cm to about 1.4 m above the seafloor. Recorded data for each elevation consists of 130 s of image pairs recorded at 1 Hz. After processing, the data at each elevation consist of 130 instantaneous spatial velocity distributions within the sample area. The vertical distribution of mean velocity indicates the presence of large-scale shear even at the highest measurement station. The flow also undergoes variations at timescales longer than the present data series.
Spatial spectra of the energy and dissipation are calculated from individual vector maps. The data extend well beyond the peak in the dissipation spectrum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range. The vector maps are also patched together to generate extended velocity distributions using the Taylor hypothesis. Spectra calculated from the extended data cover about three decades in wavenumber space. For the overlapping range the extended spectra show small differences from those determined using the instantaneous distributions. Use of the Taylor hypothesis causes “contamination” of the extended spectra with surface waves. Nevetheless, the results still indicate that the turbulence is also anisotropic at low wavenumbers (energy containing eddies). The vertical component of velocity fluctuations at energy containing scales is significantly damped as the bottom is approached, while the horizontal component maintains a similar energy level at all elevations.
Different methods of estimating the turbulent energy dissipation are compared. Several of these methods are possible only with 2D data, such as that provided by PIV, including a “direct” method, which is based on measured components of the deformation tensor. Estimates based on assumptions of isotropy are typically larger than those based on the direct method (using available velocity gradients and least number of assumptions), but the differences vary from 30% to 100%.
Abstract
Six sets of particle image velocimetry (PIV) data from the bottom boundary layer of the coastal ocean are examined. The data represent periods when the mean currents are higher, of the same order, and much weaker than the wave-induced motions. The Reynolds numbers based on the Taylor microscale (Re λ ) are 300–440 for the high, 68–83 for the moderate, and 14–37 for the weak mean currents. The moderate–weak turbulence levels are typical of the calm weather conditions at the LEO-15 site because of the low velocities and limited range of length scales. The energy spectra display substantial anisotropy at moderate to high wavenumbers and have large bumps at the transition from the inertial to the dissipation range. These bumps have been observed in previous laboratory and atmospheric studies and have been attributed to a bottleneck effect. Spatial bandpass-filtered vorticity distributions demonstrate that this anisotropy is associated with formation of small-scale, horizontal vortical layers. Methods for estimating the dissipation rates are compared, including direct estimates based on all of the gradients available from 2D data, estimates based on gradients of one velocity component, and those obtained from curve fitting to the energy spectrum. The estimates based on vertical gradients of horizontal velocity are higher and show better agreement with the direct results than do those based on horizontal gradients of vertical velocity. Because of the anisotropy and low turbulence levels, a −5/3 line-fit to the energy spectrum leads to mixed results and is especially inadequate at moderate to weak turbulence levels. The 2D velocity and vorticity distributions reveal that the flow in the boundary layer at moderate speeds consists of periods of “gusts” dominated by large vortical structures separated by periods of more quiescent flows. The frequency of these gusts increases with Re λ , and they disappear when the currents are weak. Conditional sampling of the data based on vorticity magnitude shows that the anisotropy at small scales persists regardless of vorticity and that most of the variability associated with the gusts occurs at the low-wave-number ends of the spectra. The dissipation rates, being associated with small-scale structures, do not vary substantially with vorticity magnitude. In stark contrast, almost all the contributions to the Reynolds shear stresses, estimated using structure functions, are made by the high- and intermediate-vorticity-magnitude events. During low vorticity periods the shear stresses are essentially zero. Thus, in times with weak mean flow but with wave orbital motion, the Reynolds stresses are very low. Conditional sampling based on phase in the wave orbital cycle does not show any significant trends.
Abstract
Six sets of particle image velocimetry (PIV) data from the bottom boundary layer of the coastal ocean are examined. The data represent periods when the mean currents are higher, of the same order, and much weaker than the wave-induced motions. The Reynolds numbers based on the Taylor microscale (Re λ ) are 300–440 for the high, 68–83 for the moderate, and 14–37 for the weak mean currents. The moderate–weak turbulence levels are typical of the calm weather conditions at the LEO-15 site because of the low velocities and limited range of length scales. The energy spectra display substantial anisotropy at moderate to high wavenumbers and have large bumps at the transition from the inertial to the dissipation range. These bumps have been observed in previous laboratory and atmospheric studies and have been attributed to a bottleneck effect. Spatial bandpass-filtered vorticity distributions demonstrate that this anisotropy is associated with formation of small-scale, horizontal vortical layers. Methods for estimating the dissipation rates are compared, including direct estimates based on all of the gradients available from 2D data, estimates based on gradients of one velocity component, and those obtained from curve fitting to the energy spectrum. The estimates based on vertical gradients of horizontal velocity are higher and show better agreement with the direct results than do those based on horizontal gradients of vertical velocity. Because of the anisotropy and low turbulence levels, a −5/3 line-fit to the energy spectrum leads to mixed results and is especially inadequate at moderate to weak turbulence levels. The 2D velocity and vorticity distributions reveal that the flow in the boundary layer at moderate speeds consists of periods of “gusts” dominated by large vortical structures separated by periods of more quiescent flows. The frequency of these gusts increases with Re λ , and they disappear when the currents are weak. Conditional sampling of the data based on vorticity magnitude shows that the anisotropy at small scales persists regardless of vorticity and that most of the variability associated with the gusts occurs at the low-wave-number ends of the spectra. The dissipation rates, being associated with small-scale structures, do not vary substantially with vorticity magnitude. In stark contrast, almost all the contributions to the Reynolds shear stresses, estimated using structure functions, are made by the high- and intermediate-vorticity-magnitude events. During low vorticity periods the shear stresses are essentially zero. Thus, in times with weak mean flow but with wave orbital motion, the Reynolds stresses are very low. Conditional sampling based on phase in the wave orbital cycle does not show any significant trends.
